Metformin in pregnancy: what you need to know

Metformin is one of the oldest and most widely used diabetes medicines in the world. If you are pregnant or planning a pregnancy, you may be wondering whether it is safe for your baby. The short version: it is one of the best-studied medicines in pregnancy, and the news is largely reassuring. But it is not the right choice for everyone, and the decision is one to make with your doctor.

Why might metformin be used in pregnancy?

Doctors use metformin in pregnancy in a few situations:

• Gestational diabetes (GDM), when diet and activity alone do not bring sugar into range. Read what causes gestational diabetes.
• Type 2 diabetes that was present before pregnancy, usually together with insulin.
• PCOS, where some women who were already on metformin to help them conceive are advised to continue for a time.

It is not recommended just to prevent gestational diabetes in women with obesity, or to control weight gain in pregnancy. Studies have not shown that it prevents diabetes in these situations.

Is metformin safe for my baby?

You may have heard that metformin is “Category B”. These letter categories were retired in the United States in 2015, because they oversimplified the evidence. What matters is what the studies actually show:

• Birth defects. Large studies of women who took metformin in the first three months of pregnancy have not shown a higher rate of birth defects. A 2024 study of women with type 2 diabetes found that continuing metformin with insulin in early pregnancy did not meaningfully increase the risk of pregnancy loss compared with switching to insulin alone.
• It crosses the placenta. Metformin reaches the baby, at levels similar to the mother’s. This is the main reason some guidelines are cautious.
• Long-term effects on children. This has been the biggest question. Some early follow-up studies suggested children exposed to metformin were slightly heavier as toddlers. More recent research, including a 2025 review of 18 studies, found no overall difference in body weight by later childhood. In children of mothers with PCOS, however, a few studies found slightly higher weight, so the evidence for this group is less certain. Overall the findings are mostly reassuring, but researchers continue to follow these children.

How does metformin compare with insulin?

Insulin does not cross the placenta and has been used in pregnancy for decades, which is why the American Diabetes Association still lists it as the preferred medicine for diabetes in pregnancy. Metformin is an accepted alternative or add-on, and in India it is widely used by obstetricians and diabetologists, especially when insulin is difficult to manage.

In the large MiG trial of women with gestational diabetes:

• Women on metformin gained less weight than those on insulin.
• Their babies had less severe low sugar after birth.
• Most women said they would prefer metformin in a future pregnancy.
• But about 46% needed insulin added to reach their sugar targets.
• Preterm birth was slightly more common in the metformin group, though most were late preterm births.

For women with type 2 diabetes, the MiTy trial added metformin or a dummy tablet to insulin. Metformin improved sugar control and led to less weight gain and fewer very large babies. However, more babies were small for their age. At two years, the children’s growth was similar in both groups. So metformin may not suit women whose baby is already growing small, and your doctor will consider scans of your baby’s growth.

What about metformin and PCOS in pregnancy?

Women with PCOS have higher risks of gestational diabetes, high blood pressure and preterm birth. Some studies suggest continuing metformin may slightly lower the risk of late miscarriage and preterm birth, but it does not reliably prevent gestational diabetes. Practice varies. Many doctors stop metformin once pregnancy is confirmed, while others continue it for selected women. If you are on metformin and find out you are pregnant, contact your doctor soon, but do not simply stop or change your medicine on your own. For PCOS treatment outside pregnancy, read about the 21-day pill pack for PCOS.

Is metformin safe while breastfeeding?

Yes. Only small amounts pass into breast milk, and metformin is considered compatible with breastfeeding. Many women with type 2 diabetes go back to metformin after delivery. Women who had gestational diabetes usually come off all diabetes medicines after birth, but they need a sugar test a few weeks after delivery and regular checks after that.

Are there side effects?

The common side effects are the same as outside pregnancy: nausea, loose stools, wind and a metallic taste. These are usually worse in the first few weeks. Taking it with meals, and the extended-release form, often help; your doctor will advise. Morning sickness can make side effects harder to bear, so tell your doctor if you are struggling.

How will my sugar be checked?

Whichever treatment you use, the goal is the same: keeping your sugar within the targets your team sets, measured before and after meals. Indian glucometers show readings in mg/dL; if you are reading foreign advice in mmol/L, the blood sugar unit converter helps. HbA1c is less reliable in pregnancy, so finger-prick or sensor readings are the main guide.

What should you ask your doctor?

• Is metformin, insulin or both right for my type of diabetes?
• What are my sugar targets before and after meals?
• How will we monitor my baby’s growth?
• What happens to my medicines after delivery and during breastfeeding?
• When should I have a sugar test after the baby is born?

If you have diabetes or PCOS and are planning a pregnancy, the best time to see an endocrinologist is before you conceive. Book a consultation to plan ahead.